This invention relates to the polishing of surfaces of sapphire or similar materials to remove microscopic surface imperfections and particularly to a process for polishing such surfaces, regardless of their shape, by exposing such surfaces to a substantially static environment of hydrogen.
In the fabrication of a variety of optical and electronic devices, bodies of single crystal sapphire or polycrystalline alumina or the like, having at least one surface which is as smooth as possible, are required. Examples of such requirements are dielectric substrates for semiconductor devices and transparent envelopes for use in high temperature gas discharge lamps and lasers.
Where the desired surface is flat, mechanical polishing techniques have been used in the prior art. Thus, the desired size body is cut from a larger single crystal or polycrystalline body and the desired surface is subjected to a mechanical grinding operation to reduce such surface to the "ground" (opaque) state. The surface is then polished with diamond paste or a paste of silicon carbide and buffed with an alumina powder.
By careful attention to technique, surface finishes having no scratches or other surface finishes visible at magnifications of 400.times. may be obtained. In fact, surface finishes of one microinch can be obtained with some degree of certainty but at great expense in terms of time as well as money.
Although mechanical polishing techniques may be technically feasible for flat surfaces, the problems involved in applying such techniques to the interior surfaces of cylindrical bodies, such as lamp envelopes, for example, become prohibitive. Where the body to be polished is made of soft material or a material which is hygroscopic, the above-described mechanical polishing techniques present further problems.
Thus, in the prior art, a number of chemical polishing techniques have been developed based on the use of liquid, vapor or gaseous etchants. All of such techniques have included the steps of heating the body to an elevated temperature below the melting point thereof and then subjecting the surface to be polished to a flowing etchant in an attempt to remove the surface imperfections.
The use of liquid etchants has been found to be generally undesirable since they tend to leave residues on the surface which are difficult to remove. Furthermore, liquid etching processes are inherently non-uniform and are difficult to carry out since special equipment is required to handle the flow of liquid etchant. Finally, since the body must be placed in and removed from the liquid etching equipment, in addition to subjecting it to cleaning steps, contamination of the polished surface cannot be avoided.
The beneficial effects of firing polycrystalline bodies of alumina in hydrogen have long been known as represented by U.S. Pat. No. 3,026,177. Thus, the use of flowing hydrogen as a gaseous etchant has been tried in an attempt to avoid the disadvantages of mechanical and liquid chemical polishing techniques. However, the etch rate of hydrogen was found to be so small that this approach is generally accepted as being impractical.
Thus, according to the teaching of U.S. Pat. No. 3,243,323 a polishing technique comprising the steps of heating the body to between 700.degree. and 850.degree. C. and then passing an etchant comprising a mixture of hydrogen and hydrogen chloride gases over the surface to be polished was proposed. Similarly, according to the teaching of U.S. Pat. No. 3,366,520, it has been proposed to utilize a flowing stream of hydrogen iodide in a carrier gas as the etchant at temperatures of 890.degree. to 1300.degree. C. Another proposal taught in U.S. Pat. No. 3,392,069 is to use a flowing gaseous mixture of hydrogen, hydrogen chloride and a chloride of the body material as the etchant at a temperature between 50.degree. and 500.degree. C. below the melting point of the body. More recently, it has been proposed in U.S. Pat. No. 3,546,036 to use a flowing gas containing fluorides such as SF.sub.4 and SF.sub.6 as the etchant at temperatures between 1300.degree. and 1600.degree. C.
However, all of such proposals have the disadvantage that it is difficult to accurately control the etch rate over the surface of the body, particularly where the body is polycrystalline or the surface is other than a flat plane. In the first place, perfectly homogeneous mixtures of the gases are difficult, if not impossible, to obtain and maintain and in the second place a constant and homogeneous flow of gases over the surface to be polished is impossible due to the very presence of the surface defects which are to be removed even where the surface is a flat planar surface rather than some more complex shape.
It is an object of this invention to provide a surface-polishing technique for sapphire and the like which is equally effective regardless of the shape of surface to be polished.
It is a further object of this invention to provide a surface-polishing technique for sapphire and the like which does not depend on flowing fluids or mixtures of fluids.
It is another object of this invention to provide a surface-polishing technique for sapphire and the like capable of removing all surface imperfections visible at magnifications of at least 1000.times. in a process time of one hour or less.
It is yet another object of this invention to provide a surface-polishing technique for sapphire and the like which will not contaminate the polished surface.
Briefly, the method of surface-polishing bodies of sapphire and the like according to the teaching of this invention comprises the steps of heating said body to a temperature of about 1900.degree. C. and exposing at least one surface of said body to substantially static hydrogen gas for about one hour while maintaining said temperature of about 1900.degree. C.